For separating a fluid flow of liquid and gas several different devices may be used. One major class is pure settling devices comprising relatively large gas-liquid separator tanks for receiving mixed fluid flows and stratifying the constituents. At the inlet a diffuser reduces the fluid flow velocity, whereby the heavier fluids, usually liquids, are collected by gravity in a sump and led to a liquid outlet. Stratified oil and water may be drained at different elevation levels. Gases are separated from the liquids due to gravity and be assembled above the liquid surface, and led to a gas outlet. In the separated gas flow a coalescing mesh may be arranged for catching droplets. The so formed droplets on the mesh may be collected by drain channels in or below the mesh and led down to the liquid sump. At low gas flow rates the liquid may usually be removed but at high gasflow rates there is a risk that liquid may be carried over the mesh. A high gas flow rate may also incur flooding of the mesh resulting in undesired pressure peaks. A major problem of gas-liquid separator tanks is the large required volume and the time usually required for allowing gravitational separation. Secondary droplet formation may occur due to the gas flow through the mesh. If the allowable space for the separation to take place is confined, such as in subsea petroleum production or even downhole separation of produced fluids, a compact solution is required.
Pressure or dynamic energy of the fluid flow to be separated may be utilized in a so-called cyclone separator. The inflowing mixture of gas and liquid is set into rotation either by using a high velocity tangential inflow path or by using a set of turbine blades to set the mixed fluid flow into rotation, both in a cylindrical housing. The lower-density gas will collect at the core of the fluid cyclone and the higher density liquids such as oil or water will collect at the periphery of the flowing cyclonic fluid body. The peripherally collected fluid may form a liquid film or drops on the wall of the cyclonic housing depending on the proportion of liquid to gas. The liquid part of the flow is then removed by having it flow down the wall before collection. A significant problem is that the velocity of the gas flow will shear on the liquid which may incur re-entrainment of droplets from the separated liquid back into the gas flow.
U.S. Pat. No. 6,858,067 to Burns “Filtration vessel and method for rotary gas compressor system” describes a filtration vessel for separating lube oil droplets entrained by compressed gas from a rotary screw compressor. The gas is set into rotation on entering a vortex knockout region in the lower part of the vertical cylindrical filtration vessel, and the lighter fluids in the core of the vortex rise to the upper portion of the cylindrical vessel to pass through a static, hollow concentric coalescing filter which is arranged as a non woven fine mesh for collecting and draining off remaining droplets of lube oil from the compressed gas stream passing vertically and radially, the droplet-free gas eventually leaving through a lateral upper outlet nozzle.
Several patent publications describe the use of coalescing filters for removing droplets from gases. U.S. Pat. No. 6,251,168 to Birmingham, “High efficiency gas scrubber using combined coalescing media and centrifugal cyclone”, describes a two-stage cyclone separator tank for high-quality separation of a wellhead gas stream containing an undesired high proportion of droplets or mist. Birmingham's device may separate a gas/liquid mixture which is a one-component, two-phase system or a multi-component system. An upper secondary cyclone separator is provided with tangential inlet vanes for forming the secondary cyclone motion of the gas/liquid entering the secondary cyclone. The risk of liquid re-entrainment into the gas flow is described. A coalescing filter is arranged covering the entrances to the tangential inlet vanes for initiating droplet growth for enhancing the cyclone separation effect of the secondary stage.
U.S. Pat. No. 5,334,239 to Choe, “Passive gas separator and accumulator device” describes an in-line axially arranged cylindrical filter for being arranged on a liquid line. Helical “swirler” vanes near the inlet induce a vortex motion in the liquid, and a centrally arranged static coalescing filter entraps gas bubbles and the cyclonic motion leads the coalesced gas bubbles near the centre of the cylindrical device, and lets the liquid pass peripherally. The device is particularly suited for separating out Helium bubbles from liquid Lithium such as may arise by radiation in nuclear power plants.
Published U.S. patent application US2006/0225386 describes a method for removing gaseous components such as CO2 or H2S from a contaminated natural gas stream. The method comprises first expanding the contaminated gas stream in an expander to obtain an expanded gas stream. This may take place in a turbine expander. Secondly, part of the contaminant in the gas stream is allowed to liquefy to form a dispersion of a contaminant enriched liquid phase in a contaminant depleted gaseous phase. Thirdly, the liquid phase and the gaseous phase are led into a centrifugal separator barrel with a bundle of axis-parallel channels. The separated, contaminant enriched, liquid phase is taken out axially, at an outer radial position. The separated, contaminant depleted, gas is taken out at an inner radial position and may then be recompressed such as in a turbine compressor, and reprocessed. A disadvantage of the axis-parallel channels is that they may be overfilled and partly block the gas passage. There is thus a risk of re-entrainment of liquid into the gas flow.
U.S. Pat. No. 1,075,736 to Spiegel describes an apparatus for separating liquid particles from gases. The apparatus comprises an inlet for the gas with the liquid particles to a wider cylindrical channel with a rotating drum of wide diameter, and to an outlet of lesser diameter. The rotating drum is provided with fan blades about a tapered upstream portion. The fan blades are for guiding the gas flow radially outward into the peripheral cylindrical channel about the wide drum. Downstream the gas is then radially forced inward to the narrower axial outlet of the cylindrical channel. Fine meshed screens are fixed on the surface of the drum in the annular space about the cylinder surface of the rotating drum. The fine meshed screens are for sweeping through the wet gas flow for coalescing the liquid particles in the gas flow. The coalesced liquid is then centrifugally forced out laterally and drained off in a sump.
U.S. Pat. No. 6,640,792 describes a rotating shaft mounted coalescing filter at a vent from a crankcase. The coalescing filter separates oil droplets from the gas. The rotating coalescing filter has a peripheral gas entry from the crankcase and an axial gas outlet. Coalesced liquid is centrifuged back peripherally to the crankcase.
U.S. Pat. No. 3,045,411 describes a rotary centrifugal separator for removing entrained liquids from a flow of gaseous fluid from the crankcase to the firing chamber in an internal combustion engine. The gaseous fluid is pumped radially inwards through a rotating coalescing filter and ejects the liquid radially outwards, thereby separating the entrained liquid from the gas. WO2009099339 A1 describes a separation device or unit for separating liquid from an inlet flow which mainly contains gas, the separation device comprising a container or a pipe section with an outlet for gas from the container or the pipe section, an outlet for liquid from the container or pipe section and an inlet for the inlet flow to the container or pipe section. The separation device further comprises: a flow manifold arranged to receive and put the inlet flow in movement towards a porous pipe body extending towards the gas outlet and arranged to receive the inlet flow, wherein part of the flow is flowing through the tubular body to the gas outlet, while the remaining of the flow is flowing through the porous wall of the tubular body, and an annular space consisting of the volume between the tubular body and the container wall or pipe section, the annular space is open for gas flow towards the gas outlet.
Measurements and comparisons of separation efficiency have been published in C. Verlan (1989): “Performance evaluation of impingement gas-liquid separators in Multiphase Flow” in Proceedings of the 4th International Conference. The paper shows percentage separation efficiency versus superficial gas velocity (Ug) expressed in m/s for a velocity range between 2.0 m/s and 4.5 m/s for an air/water system, please see below with respect to FIG. 16. Further, separation efficiency is also discussed in the document “Gas/Liquid Separation Technology” by Sulzer Chemtech, and shows percentage separation efficiency versus gas load factor (GLF) values between 0.0 and 0.3 m/s, please see below for FIG. 17.